U.S. patent number 4,472,683 [Application Number 06/420,303] was granted by the patent office on 1984-09-18 for imaging apparatus using nuclear magnetic resonance.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Hideki Kohno, Kensuke Sekihara, Etsuji Yamamoto, Shinji Yamamoto.
United States Patent |
4,472,683 |
Sekihara , et al. |
September 18, 1984 |
Imaging apparatus using nuclear magnetic resonance
Abstract
An imaging apparatus using nuclear magnetic resonance is
disclosed in which both an image corresponding to the projection of
a nuclear spin distribution in a to-be-inspected object on a
two-dimensional plane and a display mark indicating a desired
measuring position are displayed on a display face of a display
device such as a CRT display, the display mark is freely moved on
the display face to be set at a desired position on the display
face, and magnetic field generating means for generating a static
magnetic field, linear gradient field, or radio frequency magnetic
field is controlled on the basis of the desired position of the
display mark to obtain an image of that cross section of the object
which is located at a position indicated by the display mark.
Inventors: |
Sekihara; Kensuke (Hachioji,
JP), Yamamoto; Etsuji (Hachioji, JP),
Kohno; Hideki (Tokyo, JP), Yamamoto; Shinji
(Hachioji, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
15430185 |
Appl.
No.: |
06/420,303 |
Filed: |
September 20, 1982 |
Foreign Application Priority Data
|
|
|
|
|
Sep 18, 1981 [JP] |
|
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56-147432 |
|
Current U.S.
Class: |
324/309; 324/300;
324/307 |
Current CPC
Class: |
G01R
33/4833 (20130101) |
Current International
Class: |
G01R
33/54 (20060101); G01R 033/08 () |
Field of
Search: |
;324/300,307,309,311,310,318 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tokar; Michael J.
Attorney, Agent or Firm: Antonelli, Terry & Wands
Claims
We claim:
1. An imaging apparatus using nuclear magnetic resonance,
comprising:
a plurality of magnetic field generqting means for applying a
static magnetic field, a gradient field having gradients of field
strength in at least two of the orthogonal three directions and a
radio frequency magnetic field to an object to be inspected;
signal detecting means for detecting a nuclear magnetic resonance
signal from said object;
means for obtaining from said nuclear magnetic resonance signal an
image corresponding to the projection of said object on a
two-dimensional plane;
display means for displaying said image corresponding to said
projection;
mark positioning means for freely moving a display mark on a
display face to set said display mark at a desired position on said
display face, said image corresponding to said projection being
displayed on said display face; and
control means for controlling at least one of said magnetic field
generating means on the basis of said display mark to obtain an
image of that cross section of said object which is located at a
position indicated by said display mark.
2. An imaging apparatus using nuclear magnetic resonance according
to claim 1, wherein an input to said control means is an angle
between a predetermined direction on said display face and said
display mark, and said magnetic field generating means for applying
said gradient field to said object is controlled by said control
means on the basis of said input.
3. An imaging apparatus using nuclear magnetic resonance according
to claim 2, wherein said magnetic field generating means for
applying said gradient field to said object includes coil means for
generating said linear gradient field and a coil driver for
supplying a current to said coil means, and said control means
controls said coil driver to control said current.
4. An imaging apparatus using nuclear magnetic resonance according
to claim 1, wherein an input to said control means is a distance in
a predetermined direction between a predetermined point on said
display face and said display mark, and one of said magnetic field
generating means for applying said static magnetic field to said
object and said magnetic field generating means for applying said
radio frequency magnetic field to said object is controlled by said
control means on the basis of said input.
5. An imaging apparatus using nuclear magnetic resonance according
to claim 4, wherein said magnetic field generating means for
applying said static magnetic field to said object includes coil
means for generating said static magnetic field and a constant
current source for supplying a constant current to said coil means,
and said control means controls said constant current source to
control the magnitude of said constant current.
6. An imaging apparatus using nuclear magnetic resonance according
to claim 4, wherein said magnetic field generating means for
applying said radio frequency magnetic field to said object
includes coil means for generating said radio frequency magnetic
field, a radio frequency generator for generating a radio frequency
signal, and a modulator for moudlating said radio frequency signal
to supply a modulated signal to said coil means, and wherein said
control means controls said radio frequency source to control the
frequency of said radio frequency signal.
7. An imaging apparatus using nuclear magnetic resonance (NMR)
comprising:
means for generating a magnetic field having gradients of field
strength at least in two of orthogonal three directions;
means for exciting NMR in an object placed in said magnetic field
and for detecting NMR signal;
means for controlling said field generating means to generate a
magnetic field having gradients of field strength in two directions
and no gradient in another direction orthogonal to said two
directions;
display means for displaying an NMR image based on detected NMR
signal with a measure for identifying a position; and
means for controlling the field generating means based on the
position of said measure in the display means.
8. A method of controlling an imaging device using nuclear magnetic
resonance (NMR) which includes first means for applying a static
field, second means for applying a gradient magnetic field having
gradients of field strength, third means for applying a rotating
magnetic field and for detecting NMR signal, and means for
displaying an image, comprising the steps of:
applying magnetic field having gradients of field strength in
orthogonal two directions with said first and second means, the
field strength in a remaining direction orthogonal to said two
orthogonal directions having no gradient;
detecting NMR signal from an object in said magnetic field by said
third means, the NMR signal representing nuclear magnetic spins
integrated in said remaining direction; and
displaying a projection image of the object projected along said
remaining direction in said display means based on said NMR signal
with a measure for identifying a position in the object.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an inspecting apparatus for
nondestructively obtaining nuclear magnetic resonance (NMR) data
such as density distribution or relaxation time distribution of
nuclear spins in an object to be inspected, by utilizing nuclear
magnetic resonance (NMR), and more particularly to an imaging
apparatus using nuclear magnetic resonance which is provided with
means for determining a measuring position in an object to be
measured.
An X-ray computed tomograph (X-ray CT) and an ultrasonic imaging
apparatus have hitherto been widely used to nondestructively
inspect the internal structure of a human body or the like. In
recent years, an attempt to make a similar inspection by utilizing
nuclear magnetic resonance has been made with success, and it has
become apparent that the inspection using nuclear magnetic
resonance can obtain information which is not given by the X-ray CT
and ultrasonic imaging apparatus.
In an imaging apparatus using nuclear magnetic resonance,
information with respect to nuclear magnetic resonance such as
density distribution or relaxation time distribution of nuclear
spins in an object is nondestructively obtained from the outside of
the object by utilizing the nuclear magnetic resonance phenomenon,
and thus a cross section of a desired measuring target in the
object is reconstructed and displayed in a manner similar to the
X-ray CT.
In such an imaging apparatus, it is necessary to previously
determine a measuring target in an object. For example, in the
X-ray CT, prior to a detailed inspection (namely, a main
inspection), an X-ray image similar to an ordinary X-ray image is
obtained, for example, in such a manner that an object is
one-dimensionally moved in the X-ray CT, and an X-ray image thus
obtained is displayed to allow a physician or the like to determine
a measuring target on the basis of the displayed X-ray image.
However, conventional imaging apparatuses using nuclear magnetic
resonance are not provided with such means, and therefore cannot
previously determine a target to be measured in an object.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an
imaging apparatus using nuclear magnetic resonance in which a
measuring position in an object can be readily determined.
In order to attain the above object, according to an aspect of the
present invention, there is provided an imaging apparatus using
nuclear magnetic resonance, in which an image corresponding to the
projection of nuclear spin distribution in an object on a
two-dimensional plane is displayed in a display device, a display
mark for indicating a desired measuring position is displayed on a
display surface for displaying the above projection image, and at
least one of magnetic field generating means for generating a
static magnetic field, gradient magnetic fields and a radio
frequency magnetic field is controlled on the basis of the position
of the display mark on the display surface so that an image of that
cross section of the object which is located at a position
indicated by the display mark, can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the construction of an embodiment
of an imaging apparatus according to the present invention together
with the coordinate system.
FIG. 2 shows coil means for generating an X-direction gradient
field.
FIG. 3 shows coil means for generating a radio frequency magnetic
field.
FIG. 4 is a diagram for explaining the operation of the computer
shown in FIG. 1.
FIG. 5 is a block diagram showing a circuit configuration of the
pulse controller shown in FIG. 4.
FIG. 6 is a diagram showing waveforms of a radio frequency magnetic
field and gradient fields for obtaining a projection image.
FIG. 7 is a block diagram showing a control system in the
embodiment.
FIG. 8 is a diagram showing a method of adjusting a magnetic
field.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 schematically shows the construction of an embodiment of an
imaging apparatus according to the present invention. In FIG. 1,
reference numeral 1 designates coil means for generating a static
magnetic field which is kept uniform with an accuracy of about
10.sup.-4 in a region where inspection is carried out, 2 an object
to be inspected (in this case, a human body), 3 coil means for
generating a radio frequency magnetic field, 4 and 5 coil means for
generating gradient fields having respective gradient of field
strength in the X- and Z-directions (directions being denoted by
arrows at a righthand portion of FIG. 1, where X-direction being
upwardly perpendicular to the plane of the sheet). The field
strength of each gradient field preferably varies substantially
linearly with the distance in the specified direction. FIG. 2 shows
a structure of the coil means 4 for generating a magnetic field
whose strength varies in the X-direction. In FIG. 2, arrows
indicate the direction of coil current. The coil means 5 for
generating a magnetic field whose strength varies in the
Z-direction is obtained by rotating the coil means 4 shown in FIG.
2 about the Y-axis through 90.degree. . Further, in FIG. 1,
reference numeral 6 designates coil means including a pair of
one-turn coils connected in a manner that a current flows through
the one-turn coils in opposite directions, for generating a
magnetic field whose strength varies in the Y-direction to
establish a gradient of field strength in the Y-direction, 7, 8 and
9 coil drivers (that is, amplifiers) for supplying currents to the
coil means 4, 5 and 6, and 10 a constant current source for
supplying a constant current to the coil means 1 for generating a
static magnetic field.
The coil means 3 for generating a radio frequency magnetic field is
used not only for generating the radio frequency magnetic field but
also for detecting a nuclear magnetic resonance signal, and may be
a saddle-shaped coil, solenoid coil, or one-turn coil. FIG. 3 shows
an example of the coil means 3.
The coil drivers 7, 8 and 9 are controlled by a computer 16. The
timing of applying the gradient field and the waveform of a coil
current for generating the gradient field are previously
determined. On the other hand, a radio frequency magnetic field for
exciting magnetic spins is generated in such a manner that a radio
frequency wave generated by a synthesizer 11 is shaped and
power-amplified by a modulator 12, and then supplied to the coil
means 3 as a coil current. The synthesizer 11 and modulator 12 are
controlled by the computer 16, and moreover the frequency of the
radio frequency wave, the timing of applying the radio frequency
magnetic field and the pulse width of a pulsive radio frequency
coil current are previously determined. The above-mentioned
gradient field and radio frequency magnetic field are applied to
the object 2 which is inserted in the coil means 1 for generating a
static magnetic field H.sub.o. A signal from the object 2 is
received by the coil means 3, amplified by an amplifier 13, and
then subjected to quadrature detection in a detector 14 to be
applied to the computer 16. The computer 16 processes the received
signal to display on a cathode ray tube display 15 an image
corresponding to a nuclear spin distribution or a relaxation time
distribution of nuclear spins. The respective coil means are shown
separately for illustrative purpose, but they may be practically
wound on a same bobbin which accomodates the object.
Next, the operation of the computer 16 will be explained. As shown
in FIG. 4, the computer 16 is made up of a central processing unit
21, an interface 22, a sequence programmer 23, a sequence
controller 24, a pulse controller 25, a gradient field controller
26 and a detector control unit 2. The interface 22 decodes
instructions from the central processing unit 21, and then
instructs a sequence control part to receive the instructions. The
sequence control part includes the sequence programmer 23 for
storing the instructions from the central processing unit 21 and
the sequence controller 24. The programmer 23 is formed of, for
example, a random access memory capable of containing 256 words
each having 32 bits. One instruction includes 32 bits, the upper 8
bits of which indicate the kind of the instruction and the lower 24
bits indicate a condition specified by the instruction. The
sequence controller 24 is operated by, for example, a clock signal
of 10 MHz, and successively reads out the contents of the sequence
programmer 23 at addresses 0, 1, 2,. . . and so on to execute the
instructions. FIG. 5 shows an example of the circuit configuration
of the pulse controller 25. Referring to FIG. 5, an instruction
from the sequence control part is latched by a latch circuit 31 to
be compared, by a comparator 32, with a signal previously stored in
a latch circuit 33. Thus, it is judged whether the instruction from
the sequence control part is an instruction for the pulse
controller 25 or not. When the instruction latched by the latch
circuit 31 agrees with the signal latched by the latch circuit 33,
the instruction is transferred to a decoder 34, to determine
read-out addresses in a random access memory 35, on the basis of
the contents of the instruction, and to deliver a trigger pulse for
indicating the start of a read-out operation. The output of the
random access memory 35 is converted by a D-A converter 36 into an
analog signal. The analog signal is applied to the modulator 12 to
modulate a radio frequency signal from the synthesizer 11.
Similarly, the contents of a random access memory included in the
gradient field controller 26 are read out on the basis of an
instruction from the sequence control part, and then converted into
an analog signal. The analog signal thus obtained is applied to a
coil driver which drives the coil means for generating the gradient
field.
As mentioned above, respective waveforms of coil currents for
generating the radio frequency magnetic field and gradient field
have been previously stored in the random access memories included
in the pulse controller 25 and gradient field controller 26, and
the contents of each of the random access memories are read out on
the basis of an instruction from the sequence control part to
obtain a coil current having a desired waveform. The frequency of
the radio frequency magnetic field is selected by appropriately
changing the frequency of the output of the synthesizer 11 under
the control of the computer 16. If necessary, the frequency of the
radio frequency magnetic field may be changed in such a manner that
a control signal for selecting the frequency of the output of the
synthesizer 11 is stored in a random access memory and the
frequency of the output of the synthesizer is changed in accordance
with the control signal.
In the detector control unit 27, an analog signal which is detected
by the coil means 3 on the basis of an instruction from the
sequence control part, is converted into a digital signal at a
predetermined sampling period, and the digital signals thus
obtained are summed up simultaneously with the sampling operation.
The results of summation performed in the detector control unit 27
are applied to the central processing unit through the interface
22, to be Fourier-transformed and then subjected to the processing
for forming an image.
According to the present embodiment, prior to a main inspection for
displaying an image of the cross section of a desired measuring
target in an object, an image corresponding to the projection of a
nuclear spin distribution in the object on a two-dimensional plane
is obtained by the same apparatus as used in the main inspection
and displayed on the cathode ray tube, to enable a physician or the
like to determine the desired measuring target on the basis of the
above projection image.
Now, an inspecting procedure for a human body will be explained.
Let us assume that the human body 2 lies on a bed 20.
Referring to FIG. 1, let us consider the case where an image
indicating the projection of a nuclear spin distribution in the
human body 2 on the X-Y plane is utilized to determine a measuring
cross section of the human body 2 parallel to the X-Z plane and to
obtain an image of a nuclear spin distribution or relaxation time
distribution of nuclear spins in the cross section, by way of
example.
There are several possible ways for obtaining the above-mentioned
image indicating the projection of a distribution on the X-Y plane.
Of these possibilities, an improved version of the echo-planar
method proposed by P. Mansfield (Journal of Magnetic Resonance,
Vol. 29, 1978, pp 353 to 373) will be explained below.
FIG. 6 shows a measuring sequence in the improved version.
Referring to FIG. 6, a 90.degree. pulse from the coil means 3 for
generating a radio frequency magnetic field is applied to the whole
of the human body 2 which is an object to be inspected.
Immediately after the application of the 90.degree. pulse, the
observation of a free induction decay signal is started.
Incidentally, nuclear spins within a body placed in a static
magnetic field are parallel to the static field, on the whole. A
pulsive radio frequency magnetic field for rotating the above
nuclear spins through 90.degree. is called a 90.degree. pulse. A
width A of the 90.degree. pulse must be sufficiently narrow to
rotate nuclear spins in the whole of the field of view through
90.degree.. Symbols G.sub.y and G.sub.x in FIG. 6 designate
magnitudes of gradients of the field strength in the Y- and
X-directions, and the gradients G.sub.y and G.sub.x are given by
the coil means 6 and 4, respectively. The value of G.sub.y is
positive during a first period .tau. following the 90.degree.
pulse, and then becomes negative or positive alternately at
intervals of 2.tau. ending with a positive pulse of duration .tau.
. The value of G.sub.x is kept constant during an observation
period. As mentioned previously, the waveforms of coil currents for
genrating the radio frequency magnetic field and the gradient field
(in respective directions) have been previously stored in the
random access memories included in the pulse controller 25 and
gradient field controller 26. Respective contents of these random
access memories are read out under the control of the computer to
drive the synthesizer 11, modulator 12, and coil drivers 7 and 9,
and thus desired magnetic field with specific gradient can be
generated.
A signal detected by the coil means 3 is led to the detector 14
through the amplifier 13 to be subjected to quadrature detection,
and then applied to the computer 16 to be Fourier-transformed and
subjected to the processing for forming an image.
An image indicating the projection of a density distribution of
nuclear spins on the X-Y plane can be reconstructed from the
envelope of functions which are obtained by Fourier-transforming
the free induction decay signals detected in accordance with the
above-mentioned sequence. In the echo-planar technique, when the
length of the field of view in the direction of the X-axis is
expressed by l.sub.x, it is required to satisfy the following
relation:
l.sub.x G.sub.x =G.sub.y.
The above-mentioned method used in the present embodiment is
different from the method described in the previously-mentioned
publication in that a sequence for selecting a specified slice
plane perpendicular to the Z-direction shown in FIG. 1 is omitted
in the method used in the present invention. This enables one to
obtain a projection image. The image indicating the projection of a
nuclear spin distribution on the X-Y plane has the following
advantages in determining the position of a cross section to be
measured. That is, a two-dimensional image obtained by the method
used in the present embodiment corresponds to the projection of a
three-dimensional nuclear spin distribution on the X-Y plane, and
therefore a point on the image indicates the sum of nuclear spin
signals in the Z-direction. Accordingly, the image has an extremely
high signal-to-noise ratio. Further, in the case where an image of
a specified cross section perpendicular to the Z-axis is used in
place of the above image, the shape of those organs in human body
which are distributed in the Z-direction, varies greatly with the
value of the Z-coordinate of the specified cross section.
Therefore, the image of the specified cross section is unsuitable
for determining a cross section to be measured in the main
inspection. The image obtained by the method used in the present
embodiment includes integrated values in the Z-direction, and
therefore can eliminate such a problem, i.e. can be used
advantageously as compared with the image of a specified cross
section perpendicular to the Z-direction.
Next, explanation will be made on determination of a to-be-measured
cross section using the above-mentioned projection image, with
reference to FIG. 7.
FIG. 7 is a block diagram showing a control system in the present
embodiment. In FIG. 7, reference numeral 15 designates a display
device such as a cathode ray tube display, and 15a a display
surface on which an image corresponding to a nuclear spin
distribution or a relaxation time distribution of nuclear spins is
displayed. As shown in FIG. 7, on the display face 15a are
displayed an image I corresponding to the projection of a nuclear
spin distribution on the X-Y plane and a linear display mark
(namely, a cursor) M which is indicated by a broken line. The
display mark M can be freely moved on the display face 15a by an
external operation, to be set at a desired position on the display
face, and the mark M indicates the position of a to-be-measured
cross section perpendicular to the X-Y plane. A mark positioning
unit 17 provided, for example, in a console is employed to set the
display mark M at a desired position on the display face 15a. The
mark operating part of the mark positioning unit 17 includes, for
example, two variable resistors. The displacement of the display
mark M in the Y-direction (that is, in the downward direction in
FIG. 7) at the display face 15a, that is, a distance L of the mark
M from the center Q of the field of view on Y-axis can be freely
set by a first one of the variable resistors, and an angle
.theta.between the display mark M and the Y-axis can be freely set
by a second variable resistor. Since the construction of a computer
16 has been explained in detail in connection with FIG. 4,
explanation will be made on only the operation of the computer for
determining a cross section to be measured. A mark positioning
signal from the mark positioning unit 17 is applied to the central
processing unit 21 through the interface 22 to be converted into a
mark display signal for displaying a linear display mark. The mark
display signal is combined with a signal for displaying the
previously-mentioned projection image. The composite signal thus
formed is supplied to the display device 15 through the interface
22 to display the projection image I and the display mark M on the
display face 15a. Further, in the central processing unit 21, an
actual length L corresponding to the length L of the mark M from
the center Q of the field of view and the angle .theta.between the
mark M and the Y-axis are calculated on the basis of the mark
positioning signal. It will be apparent that the mark M may be a
calibrated rule.
In an imaging apparatus using nuclear magnetic resonance, the
direction of a cross section to be measured can be freely selected
by adjusting the gradient of field strength, e.g. in each of a pair
of gradient fields which are orthogonal in direction of gradient of
field strength to each other. When the gradients of field strength
in the X- and Y-directions in the apparatus shown in FIG. 7 are
expressed by G.sub.x and G.sub.y, respectively, a gradient G of
field strength obtained by simultaneously applying the gradient
fields having the gradients G.sub.x and G.sub.y is given by the
following equation:
G=G.sub.x i+G.sub.y j,
where G is a vector indicating the gradient G of field strength, i
a unit vector in the X-direction, and j a unit vector in the
Y-direction. Accordingly, a cross section making an angle
.theta.with the Y-axis can be obtained by selecting the gradients
G.sub.x and G.sub.y so that an equation tan.sup.-1 (G.sub.x
/G.sub.y) =.theta.is satisfied. Such selection of the gradients
G.sub.x and G.sub.y can be made, for example, in the following
manner. That is, the ratio G.sub.x /G.sub.y is calculated using the
above equation and the angle .theta.obtained by the computer 16.
Then, currents for generating gradient fields corresponding to the
ratio G.sub.x /G.sub.y are produced in the gradient field
controller 26, to be supplied to the coil driver 7 for generating
the gradient field whose field strength varies in the X-direction
and the coil driver 9 for generating the gradient field whose field
strength varies in the Y-direction. Alternatively, currents having
a predetermined magnitude are generated by the gradient field
controller 26, and the gain of the coil drivers 7 and 9 supplied
with the currents may be controlled in accordance with the
calculated ratio G.sub.x /G.sub.y.
Further, a cross section corresponding to a straight line on the
display face 15a which is spaced apart from the center Q of the
field of view by a length L in the Y-direction, can be measured in
the following manner. That is, the gradient fields, the radio
frequency magnetic field and a receiver are adjusted so that the
image of a cross section intersecting the center of the field of
view can be obtained by an imaging system, and then the bed 20
shown in FIG. 1 is moved by an actual distance L corresponding to
the above-mentioned distance L so that the desired cross section
indicated by the cursor (namely, the display mark) M intersects the
center Q of the field of view.
The above-mentioned method may be replaced by the following second
method. In an imaging apparatus using nuclear magnetic resonance,
the position of a to-be-measured cross section is determined by the
strength H of a magnetic field previously applied to an object to
be inspected and the frequency of a radio frequency magnetic field
applied perpendicularly to the above magnetic field. The strength H
of the above magnetic field is generally expressed as follows:
where H.sub.G is a gradient field component, and H.sub.o a static
field component which is uniform throughout the magnetic field
region. Now, the second method will be explained in detail with
reference to FIG. 8. In FIG. 8, reference symbol P.sub.l designates
a cross section intersecting the center of the field of view, and
P.sub.2 a cross section which is spaced apart from the center of
the field of view by a distance L (corresponding to a distance L on
the display face of the display device 15) and is required to be
measured. In order to measure the cross section P.sub.2, when the
values of the field strength H at the cross sections P.sub.1 and
P.sub.2 are expressed by H, and H.sub.2, the frequency f.sub.2 of
the radio frequency magnetic field is set so as to satisfy the
following relation:
where .gamma.is a gyromagnetic ratio. The field strength H.sub.2
can be obtained from an equation H.sub.2 =H.sub.1 +G.sub.y L, using
the distance L and a gradient G.sub.y of field strength in the
Y-direction.
Alternatively, in the case where the frequency of the radio
frequency magnetic field is equal to f.sub.1 (=.gamma.H.sub.1 ),
that is, the radio frequency magnetic field is adjusted so that the
cross section P.sub.1 intersecting the center of the field of view
is measured, the frequency of the radio frequency magnetic field is
not changed, but the static field H.sub.o is varied so that the
field strength H at the cross section P.sub.2 becomes equal to
H.sub.1.
In the former case, the frequency of the radio frequency magnetic
field is varied by changing the frequency of the output of the
synthesizer 11 under the control of the computer. In the latter
case, the strength H.sub.o of the static magnetic field is varied
in such a manner that a current supplied to the coil means 1 for
generating the static magnetic field is varied, for example, by
controlling a variable resistor in the constant current source
10.
In the above-mentioned explanation of the present embodiment, the
improved version of the echoplanar method is employed to obtain the
projection image. However, the present invention is not limited to
such a method.
As has been explained in the foregoing, according to the present
invention, an image corresponding to the projection of a nuclear
spin distribution in an object on a two-dimensional plane is
displayed on a display device, and means for generating a static
magnetic field, gradient fields or a radio frequency magnetic field
is controlled in accordance with the position of a display mark on
a display face for displaying the above image. Accordingly, the
present invention can provide an imaging apparatus in which the
position of a to-be-measured target in an object can be readily
determined.
* * * * *